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		<title>The Equality Relation Revisited</title>
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    <ul class="crumbs"><li><a href="../index.html">Home</a></li><li><a href="../inform7n.html">Inform7</a></li><li><a href="index.html">assertions</a></li><li><a href="index.html#8">Chapter 8: Predicates</a></li><li><b>The Equality Relation Revisited</b></li></ul></div>
<p class="purpose">To define how equality behaves in the Inform language.</p>

<ul class="toc"><li><a href="8-terr.html#SP1">&#167;1. Additional details</a></li><li><a href="8-terr.html#SP2">&#167;2. Typechecking</a></li><li><a href="8-terr.html#SP5">&#167;5. Assertion</a></li><li><a href="8-terr.html#SP7">&#167;7. Compilation</a></li></ul><hr class="tocbar">

<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>&#167;1. Additional details.</b></p>

<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">EqualityDetails::start</span><button class="popup" onclick="togglePopup('usagePopup1')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup1">Usage of <span class="code-font"><span class="function-syntax">EqualityDetails::start</span></span>:<br/>Assertions Module - <a href="1-am.html#SP2">&#167;2</a></span></button><span class="plain-syntax">(</span><span class="reserved-syntax">void</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">METHOD_ADD</span><span class="plain-syntax">(</span><span class="identifier-syntax">equality_bp_family</span><span class="plain-syntax">, </span><span class="identifier-syntax">TYPECHECK_BPF_MTID</span><span class="plain-syntax">, </span><a href="8-terr.html#SP2" class="function-link"><span class="function-syntax">EqualityDetails::typecheck</span></a><span class="plain-syntax">);</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">METHOD_ADD</span><span class="plain-syntax">(</span><span class="identifier-syntax">equality_bp_family</span><span class="plain-syntax">, </span><span class="identifier-syntax">ASSERT_BPF_MTID</span><span class="plain-syntax">, </span><a href="8-terr.html#SP5" class="function-link"><span class="function-syntax">EqualityDetails::assert</span></a><span class="plain-syntax">);</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">METHOD_ADD</span><span class="plain-syntax">(</span><span class="identifier-syntax">equality_bp_family</span><span class="plain-syntax">, </span><span class="identifier-syntax">SCHEMA_BPF_MTID</span><span class="plain-syntax">, </span><a href="8-terr.html#SP7" class="function-link"><span class="function-syntax">EqualityDetails::schema</span></a><span class="plain-syntax">);</span>

<span class="plain-syntax">    </span><span class="identifier-syntax">METHOD_ADD</span><span class="plain-syntax">(</span><span class="identifier-syntax">empty_bp_family</span><span class="plain-syntax">, </span><span class="identifier-syntax">TYPECHECK_BPF_MTID</span><span class="plain-syntax">, </span><a href="8-terr.html#SP4" class="function-link"><span class="function-syntax">EqualityDetails::typecheck_empty</span></a><span class="plain-syntax">);</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">METHOD_ADD</span><span class="plain-syntax">(</span><span class="identifier-syntax">empty_bp_family</span><span class="plain-syntax">, </span><span class="identifier-syntax">ASSERT_BPF_MTID</span><span class="plain-syntax">, </span><a href="8-terr.html#SP6" class="function-link"><span class="function-syntax">EqualityDetails::assert_empty</span></a><span class="plain-syntax">);</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">METHOD_ADD</span><span class="plain-syntax">(</span><span class="identifier-syntax">empty_bp_family</span><span class="plain-syntax">, </span><span class="identifier-syntax">SCHEMA_BPF_MTID</span><span class="plain-syntax">, </span><a href="8-terr.html#SP8" class="function-link"><span class="function-syntax">EqualityDetails::schema_empty</span></a><span class="plain-syntax">);</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>&#167;2. Typechecking.</b>This is a very polymorphic relation, in that it can accept terms of almost
any kind.
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">EqualityDetails::typecheck</span><button class="popup" onclick="togglePopup('usagePopup2')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup2">Usage of <span class="code-font"><span class="function-syntax">EqualityDetails::typecheck</span></span>:<br/><a href="8-terr.html#SP1">&#167;1</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">bp_family</span><span class="plain-syntax"> *</span><span class="identifier-syntax">self</span><span class="plain-syntax">, </span><span class="identifier-syntax">binary_predicate</span><span class="plain-syntax"> *</span><span class="identifier-syntax">bp</span><span class="plain-syntax">,</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">kind</span><span class="plain-syntax"> **</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">, </span><span class="identifier-syntax">kind</span><span class="plain-syntax"> **</span><span class="identifier-syntax">kinds_required</span><span class="plain-syntax">, </span><span class="identifier-syntax">tc_problem_kit</span><span class="plain-syntax"> *</span><span class="identifier-syntax">tck</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">LOGIF</span><span class="plain-syntax">(</span><span class="identifier-syntax">MATCHING</span><span class="plain-syntax">, </span><span class="string-syntax">"Typecheck %u '==' %u\n"</span><span class="plain-syntax">, </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1]);</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">K_understanding</span><span class="plain-syntax">) &amp;&amp; (</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">K_understanding</span><span class="plain-syntax">)) &amp;&amp;</span>
<span class="plain-syntax">            (</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">K_text</span><span class="plain-syntax">))) {</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">LOGIF</span><span class="plain-syntax">(</span><span class="identifier-syntax">MATCHING</span><span class="plain-syntax">, </span><span class="string-syntax">"No!\n"</span><span class="plain-syntax">);</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">StandardProblems::tcp_problem</span><span class="plain-syntax">(</span><span class="identifier-syntax">_p_</span><span class="plain-syntax">(</span><span class="identifier-syntax">PM_TextIsNotTopic</span><span class="plain-syntax">), </span><span class="identifier-syntax">tck</span><span class="plain-syntax">,</span>
<span class="plain-syntax">            </span><span class="string-syntax">"though they look the same, because both are written in double "</span>
<span class="plain-syntax">            </span><span class="string-syntax">"quotes, text values can't in fact be used as topics, so it's "</span>
<span class="plain-syntax">            </span><span class="string-syntax">"impossible to store this piece of text in that location."</span><span class="plain-syntax">);</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">NEVER_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    }</span>

<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">PluginCalls::typecheck_equality</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1]))</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">ALWAYS_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Kinds::Behaviour::is_object</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0])) &amp;&amp;</span>
<span class="plain-syntax">        (</span><span class="identifier-syntax">Properties::can_name_coincide_with_kind</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1])))</span>
<span class="plain-syntax">        </span><span class="named-paragraph-container code-font"><a href="8-terr.html#SP2_1" class="named-paragraph-link"><span class="named-paragraph">Apply rule for "is" applied to an object and a value</span><span class="named-paragraph-number">2.1</span></a></span>
<span class="plain-syntax">    </span><span class="reserved-syntax">else</span><span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">K_understanding</span><span class="plain-syntax">) &amp;&amp; (</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">K_understanding</span><span class="plain-syntax">)) &amp;&amp;</span>
<span class="plain-syntax">            (</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">K_snippet</span><span class="plain-syntax">)))</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">ALWAYS_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">else</span><span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">K_understanding</span><span class="plain-syntax">) &amp;&amp; (</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">K_understanding</span><span class="plain-syntax">)) &amp;&amp;</span>
<span class="plain-syntax">            (</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">K_snippet</span><span class="plain-syntax">)))</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">ALWAYS_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">else</span><span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">K_text</span><span class="plain-syntax">)) &amp;&amp;</span>
<span class="plain-syntax">            (</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">K_response</span><span class="plain-syntax">)))</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">ALWAYS_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">else</span>
<span class="plain-syntax">        </span><span class="named-paragraph-container code-font"><a href="8-terr.html#SP2_2" class="named-paragraph-link"><span class="named-paragraph">Allow comparison only where left domain and right domain are not disjoint</span><span class="named-paragraph-number">2.2</span></a></span><span class="plain-syntax">;</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">ALWAYS_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP2_1" class="paragraph-anchor"></a><b>&#167;2.1. </b>This case is only separated in order to provide a better problem message
for a fairly common mistake:
</p>

<p class="commentary"><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Apply rule for "is" applied to an object and a value</span><span class="named-paragraph-number">2.1</span></span><span class="comment-syntax"> =</span>
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="plain-syntax">    </span><span class="identifier-syntax">property</span><span class="plain-syntax"> *</span><span class="identifier-syntax">prn</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Properties::property_with_same_name_as</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1]);</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">prn</span><span class="plain-syntax"> == </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">tck</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">log_to_I6_text</span><span class="plain-syntax">)</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">LOG</span><span class="plain-syntax">(</span><span class="string-syntax">"Comparison of object with %u value\n"</span><span class="plain-syntax">, </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1]);</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">Problems::quote_kind</span><span class="plain-syntax">(4, </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1]);</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">StandardProblems::tcp_problem</span><span class="plain-syntax">(</span><span class="identifier-syntax">_p_</span><span class="plain-syntax">(</span><span class="identifier-syntax">PM_NonPropertyCompared</span><span class="plain-syntax">), </span><span class="identifier-syntax">tck</span><span class="plain-syntax">,</span>
<span class="plain-syntax">            </span><span class="string-syntax">"taken literally that says that an object is the same as a "</span>
<span class="plain-syntax">            </span><span class="string-syntax">"value. Maybe you intended to say that the object "</span>
<span class="plain-syntax">            </span><span class="string-syntax">"has a property - but right now %4 is not yet a property; if you "</span>
<span class="plain-syntax">            </span><span class="string-syntax">"want to use it as one, you'll need to say so. (You can turn a "</span>
<span class="plain-syntax">            </span><span class="string-syntax">"kind of value - say, 'colour' - into a property by writing - "</span>
<span class="plain-syntax">            </span><span class="string-syntax">"say - 'A thing has a colour.')"</span><span class="plain-syntax">);</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">NEVER_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    }</span>
</pre>
<ul class="endnotetexts"><li>This code is used in <a href="8-terr.html#SP2">&#167;2</a>.</li></ul>
<p class="commentary firstcommentary"><a id="SP2_2" class="paragraph-anchor"></a><b>&#167;2.2. </b>With comparisons there is no restriction on the two kinds except that they
must match each other; \({\it is}(t, s)\) is allowed if \(K(t)\subseteq K(s)\) or
vice versa. So rules and rulebooks are comparable, for instance, but numbers
and scenes are not.
</p>

<p class="commentary"><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Allow comparison only where left domain and right domain are not disjoint</span><span class="named-paragraph-number">2.2</span></span><span class="comment-syntax"> =</span>
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><a href="8-terr.html#SP3" class="function-link"><span class="function-syntax">EqualityDetails::both_terms_of_same_construction</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">CON_rule</span><span class="plain-syntax">))</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">ALWAYS_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><a href="8-terr.html#SP3" class="function-link"><span class="function-syntax">EqualityDetails::both_terms_of_same_construction</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">CON_rulebook</span><span class="plain-syntax">))</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">ALWAYS_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><a href="8-terr.html#SP3" class="function-link"><span class="function-syntax">EqualityDetails::both_terms_of_same_construction</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">CON_activity</span><span class="plain-syntax">))</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">ALWAYS_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Kinds::compatible</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1]) == </span><span class="identifier-syntax">NEVER_MATCH</span><span class="plain-syntax">) &amp;&amp;</span>
<span class="plain-syntax">        (</span><span class="identifier-syntax">Kinds::compatible</span><span class="plain-syntax">(</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0]) == </span><span class="identifier-syntax">NEVER_MATCH</span><span class="plain-syntax">)) {</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">tck</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">log_to_I6_text</span><span class="plain-syntax">)</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">LOG</span><span class="plain-syntax">(</span><span class="string-syntax">"Unable to compare %u with %u\n"</span><span class="plain-syntax">, </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">[1]);</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">NEVER_MATCH_SAYING_WHY_NOT</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    }</span>
</pre>
<ul class="endnotetexts"><li>This code is used in <a href="8-terr.html#SP2">&#167;2</a>.</li></ul>
<p class="commentary firstcommentary"><a id="SP3" class="paragraph-anchor"></a><b>&#167;3. </b></p>

<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">EqualityDetails::both_terms_of_same_construction</span><button class="popup" onclick="togglePopup('usagePopup3')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup3">Usage of <span class="code-font"><span class="function-syntax">EqualityDetails::both_terms_of_same_construction</span></span>:<br/><a href="8-terr.html#SP2_2">&#167;2.2</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">kind</span><span class="plain-syntax"> *</span><span class="identifier-syntax">k0</span><span class="plain-syntax">, </span><span class="identifier-syntax">kind</span><span class="plain-syntax"> *</span><span class="identifier-syntax">k1</span><span class="plain-syntax">, </span><span class="identifier-syntax">kind_constructor</span><span class="plain-syntax"> *</span><span class="identifier-syntax">cons</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Kinds::get_construct</span><span class="plain-syntax">(</span><span class="identifier-syntax">k0</span><span class="plain-syntax">) == </span><span class="identifier-syntax">cons</span><span class="plain-syntax">) &amp;&amp; (</span><span class="identifier-syntax">Kinds::get_construct</span><span class="plain-syntax">(</span><span class="identifier-syntax">k1</span><span class="plain-syntax">) == </span><span class="identifier-syntax">cons</span><span class="plain-syntax">))</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP4" class="paragraph-anchor"></a><b>&#167;4. </b>The never-holding relation is simpler: anything can be hypothetically related to
anything else (except of course that it always is not).
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">EqualityDetails::typecheck_empty</span><button class="popup" onclick="togglePopup('usagePopup4')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup4">Usage of <span class="code-font"><span class="function-syntax">EqualityDetails::typecheck_empty</span></span>:<br/><a href="8-terr.html#SP1">&#167;1</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">bp_family</span><span class="plain-syntax"> *</span><span class="identifier-syntax">self</span><span class="plain-syntax">, </span><span class="identifier-syntax">binary_predicate</span><span class="plain-syntax"> *</span><span class="identifier-syntax">bp</span><span class="plain-syntax">,</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">kind</span><span class="plain-syntax"> **</span><span class="identifier-syntax">kinds_of_terms</span><span class="plain-syntax">, </span><span class="identifier-syntax">kind</span><span class="plain-syntax"> **</span><span class="identifier-syntax">kinds_required</span><span class="plain-syntax">, </span><span class="identifier-syntax">tc_problem_kit</span><span class="plain-syntax"> *</span><span class="identifier-syntax">tck</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">ALWAYS_MATCH</span><span class="plain-syntax">;</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP5" class="paragraph-anchor"></a><b>&#167;5. Assertion.</b>In general values differ, and cannot be equated by fiat. But an exception is
setting a global variable.
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">EqualityDetails::assert</span><button class="popup" onclick="togglePopup('usagePopup5')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup5">Usage of <span class="code-font"><span class="function-syntax">EqualityDetails::assert</span></span>:<br/><a href="8-terr.html#SP1">&#167;1</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">bp_family</span><span class="plain-syntax"> *</span><span class="identifier-syntax">self</span><span class="plain-syntax">, </span><span class="identifier-syntax">binary_predicate</span><span class="plain-syntax"> *</span><span class="identifier-syntax">bp</span><span class="plain-syntax">,</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">inference_subject</span><span class="plain-syntax"> *</span><span class="identifier-syntax">infs0</span><span class="plain-syntax">, </span><span class="identifier-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">spec0</span><span class="plain-syntax">,</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">inference_subject</span><span class="plain-syntax"> *</span><span class="identifier-syntax">infs1</span><span class="plain-syntax">, </span><span class="identifier-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">spec1</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Lvalues::is_actual_NONLOCAL_VARIABLE</span><span class="plain-syntax">(</span><span class="identifier-syntax">spec0</span><span class="plain-syntax">)) {</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">nonlocal_variable</span><span class="plain-syntax"> *</span><span class="identifier-syntax">q</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Node::get_constant_nonlocal_variable</span><span class="plain-syntax">(</span><span class="identifier-syntax">spec0</span><span class="plain-syntax">);</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">allowed</span><span class="plain-syntax"> = </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">prevailing_mood</span><span class="plain-syntax"> != </span><span class="identifier-syntax">UNKNOWN_CE</span><span class="plain-syntax">) &amp;&amp; (</span><span class="identifier-syntax">prevailing_mood</span><span class="plain-syntax"> != </span><span class="identifier-syntax">LIKELY_CE</span><span class="plain-syntax">))</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">allowed</span><span class="plain-syntax"> = </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">NonlocalVariables::is_constant</span><span class="plain-syntax">(</span><span class="identifier-syntax">q</span><span class="plain-syntax">)) &amp;&amp; (</span><span class="identifier-syntax">prevailing_mood</span><span class="plain-syntax"> == </span><span class="identifier-syntax">CERTAIN_CE</span><span class="plain-syntax">))</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">allowed</span><span class="plain-syntax"> = </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">allowed</span><span class="plain-syntax"> == </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">)</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">StandardProblems::sentence_problem</span><span class="plain-syntax">(</span><span class="identifier-syntax">Task::syntax_tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">_p_</span><span class="plain-syntax">(</span><span class="identifier-syntax">PM_CantQualifyVariableValues</span><span class="plain-syntax">),</span>
<span class="plain-syntax">                </span><span class="string-syntax">"a variable can only be given its value straightforwardly or "</span>
<span class="plain-syntax">                </span><span class="string-syntax">"qualified by 'usually'"</span><span class="plain-syntax">,</span>
<span class="plain-syntax">                </span><span class="string-syntax">"not with 'always', 'seldom' or 'never'."</span><span class="plain-syntax">);</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">else</span><span class="plain-syntax"> </span><span class="identifier-syntax">PropertyInferences::draw</span><span class="plain-syntax">(</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">NonlocalVariables::to_subject</span><span class="plain-syntax">(</span><span class="identifier-syntax">q</span><span class="plain-syntax">), </span><span class="identifier-syntax">P_variable_initial_value</span><span class="plain-syntax">, </span><span class="identifier-syntax">spec1</span><span class="plain-syntax">);</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    }</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP6" class="paragraph-anchor"></a><b>&#167;6. </b>The never-holding relation cannot be asserted true:
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">EqualityDetails::assert_empty</span><button class="popup" onclick="togglePopup('usagePopup6')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup6">Usage of <span class="code-font"><span class="function-syntax">EqualityDetails::assert_empty</span></span>:<br/><a href="8-terr.html#SP1">&#167;1</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">bp_family</span><span class="plain-syntax"> *</span><span class="identifier-syntax">self</span><span class="plain-syntax">, </span><span class="identifier-syntax">binary_predicate</span><span class="plain-syntax"> *</span><span class="identifier-syntax">bp</span><span class="plain-syntax">,</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">inference_subject</span><span class="plain-syntax"> *</span><span class="identifier-syntax">infs0</span><span class="plain-syntax">, </span><span class="identifier-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">spec0</span><span class="plain-syntax">,</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">inference_subject</span><span class="plain-syntax"> *</span><span class="identifier-syntax">infs1</span><span class="plain-syntax">, </span><span class="identifier-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">spec1</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP7" class="paragraph-anchor"></a><b>&#167;7. Compilation.</b>Since we are compiling to I6, which is itself a C-level programming
language, it looks at first as if we can compile is into <span class="extract"><span class="extract-syntax">==</span></span> when
testing equality and <span class="extract"><span class="extract-syntax">=</span></span> when asserting it: thus
</p>

<blockquote>
    <p>now the score is 10;</p>
</blockquote>

<blockquote>
    <p>if the score is 10, ...</p>
</blockquote>

<p class="commentary">would compile to <span class="extract"><span class="extract-syntax">score = 10;</span></span> and <span class="extract"><span class="extract-syntax">if (score == 10) ...</span></span> respectively.
</p>

<p class="commentary">But there are three problems with this simplistic approach to "A is B".
</p>

<ul class="items"><li>(a) Sometimes "now A is B" must set a property of A, which does not
change, rather than making A equal to B; and similarly for testing.
</li><li>(b) Sometimes A is reference to a value stored in some data structure
other than a local or global variable: for example, in "now entry 3 of
the passenger list is 208", where A is "entry 3 of the passenger list".
Access to this value is via I6 routines in the template, and the form of
what we compile has to be different depending on whether we are reading
or writing.
</li><li>(c) Sometimes the values in question are block values, that is, they are
stored as pointers to blocks of data on the heap at run-time. If we compile
"now T is X", where T is a text variable and X is some piece of text, we
cannot simply copy the pointers: T needs to hold a fresh, independent copy of
the text referred to by X.
</li></ul>
<p class="commentary">Problem (a) is easily detected by looking at the kinds of value of A and B.
To handle problems (b) and (c), we use a general framework in which the
schema is a function of both the storage class of A and the kinds of value
of both A and B.
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">EqualityDetails::schema</span><button class="popup" onclick="togglePopup('usagePopup7')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup7">Usage of <span class="code-font"><span class="function-syntax">EqualityDetails::schema</span></span>:<br/><a href="8-terr.html#SP1">&#167;1</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">bp_family</span><span class="plain-syntax"> *</span><span class="identifier-syntax">self</span><span class="plain-syntax">, </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">task</span><span class="plain-syntax">, </span><span class="identifier-syntax">binary_predicate</span><span class="plain-syntax"> *</span><span class="identifier-syntax">bp</span><span class="plain-syntax">, </span><span class="identifier-syntax">annotated_i6_schema</span><span class="plain-syntax"> *</span><span class="identifier-syntax">asch</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">kind</span><span class="plain-syntax"> *</span><span class="identifier-syntax">st</span><span class="plain-syntax">[2];</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">st</span><span class="plain-syntax">[0] = </span><span class="identifier-syntax">Cinders::kind_of_term</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">pt0</span><span class="plain-syntax">);</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">st</span><span class="plain-syntax">[1] = </span><span class="identifier-syntax">Cinders::kind_of_term</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">pt1</span><span class="plain-syntax">);</span>

<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Kinds::Behaviour::is_object</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[0])) &amp;&amp;</span>
<span class="plain-syntax">        (</span><span class="identifier-syntax">Properties::can_name_coincide_with_kind</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[1])) &amp;&amp; (</span><span class="identifier-syntax">Properties::property_with_same_name_as</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[1])))</span>
<span class="plain-syntax">        </span><span class="named-paragraph-container code-font"><a href="8-terr.html#SP7_1" class="named-paragraph-link"><span class="named-paragraph">Handle the case of setting a property of A separately</span><span class="named-paragraph-number">7.1</span></a></span><span class="plain-syntax">;</span>

<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">K_response</span><span class="plain-syntax">)) &amp;&amp; (</span><span class="identifier-syntax">Kinds::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">K_text</span><span class="plain-syntax">)))</span>
<span class="plain-syntax">        </span><span class="named-paragraph-container code-font"><a href="8-terr.html#SP7_2" class="named-paragraph-link"><span class="named-paragraph">Handle the case of setting a response separately</span><span class="named-paragraph-number">7.2</span></a></span><span class="plain-syntax">;</span>

<span class="plain-syntax">    </span><span class="reserved-syntax">switch</span><span class="plain-syntax"> (</span><span class="identifier-syntax">task</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">TEST_ATOM_TASK:</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">st</span><span class="plain-syntax">[0]) &amp;&amp; (</span><span class="identifier-syntax">st</span><span class="plain-syntax">[1]))</span>
<span class="plain-syntax">                </span><span class="identifier-syntax">Calculus::Schemas::modify</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">schema</span><span class="plain-syntax">, </span><span class="string-syntax">"%S"</span><span class="plain-syntax">,</span>
<span class="plain-syntax">                    </span><span class="identifier-syntax">EqualitySchemas::interpret_equality</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">st</span><span class="plain-syntax">[1]));</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">else</span><span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">problem_count</span><span class="plain-syntax"> == </span><span class="constant-syntax">0</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">                </span><span class="identifier-syntax">LOG</span><span class="plain-syntax">(</span><span class="string-syntax">"$0 and $0; %u and %u\n"</span><span class="plain-syntax">, &amp;(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">pt0</span><span class="plain-syntax">), &amp;(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">pt1</span><span class="plain-syntax">), </span><span class="identifier-syntax">st</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">st</span><span class="plain-syntax">[1]);</span>
<span class="plain-syntax">                </span><span class="identifier-syntax">StandardProblems::sentence_problem</span><span class="plain-syntax">(</span><span class="identifier-syntax">Task::syntax_tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">_p_</span><span class="plain-syntax">(</span><span class="identifier-syntax">BelievedImpossible</span><span class="plain-syntax">),</span>
<span class="plain-syntax">                    </span><span class="string-syntax">"that would involve comparing things which don't mean "</span>
<span class="plain-syntax">                    </span><span class="string-syntax">"anything to me"</span><span class="plain-syntax">,</span>
<span class="plain-syntax">                    </span><span class="string-syntax">"so I'm lost."</span><span class="plain-syntax">);</span>
<span class="plain-syntax">            }</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">NOW_ATOM_FALSE_TASK:</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">break</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">NOW_ATOM_TRUE_TASK:</span><span class="plain-syntax"> {</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">node_type_t</span><span class="plain-syntax"> </span><span class="identifier-syntax">storage_class</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Lvalues::get_storage_form</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">pt0</span><span class="plain-syntax">.</span><span class="identifier-syntax">constant</span><span class="plain-syntax">);</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">storage_class</span><span class="plain-syntax"> == </span><span class="identifier-syntax">UNKNOWN_NT</span><span class="plain-syntax">) &amp;&amp;</span>
<span class="plain-syntax">                (</span><span class="identifier-syntax">Kinds::get_construct</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[0]) == </span><span class="identifier-syntax">CON_property</span><span class="plain-syntax">))</span>
<span class="plain-syntax">                </span><span class="identifier-syntax">storage_class</span><span class="plain-syntax"> = </span><span class="identifier-syntax">PROPERTY_VALUE_NT</span><span class="plain-syntax">;</span>
<span class="plain-syntax">            </span><span class="named-paragraph-container code-font"><a href="8-terr.html#SP7_4" class="named-paragraph-link"><span class="named-paragraph">Make a further check that kinds permit this assignment</span><span class="named-paragraph-number">7.4</span></a></span><span class="plain-syntax">;</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">storage_class</span><span class="plain-syntax"> == </span><span class="identifier-syntax">UNKNOWN_NT</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">                </span><span class="named-paragraph-container code-font"><a href="8-terr.html#SP7_5" class="named-paragraph-link"><span class="named-paragraph">Issue problem message for being unable to set equal</span><span class="named-paragraph-number">7.5</span></a></span>
<span class="plain-syntax">                </span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">schema</span><span class="plain-syntax"> = </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">;</span>
<span class="plain-syntax">            } </span><span class="reserved-syntax">else</span><span class="plain-syntax"> {</span>
<span class="plain-syntax">                </span><span class="named-paragraph-container code-font"><a href="8-terr.html#SP7_3" class="named-paragraph-link"><span class="named-paragraph">Exceptional case of setting the "player" global variable</span><span class="named-paragraph-number">7.3</span></a></span><span class="plain-syntax">;</span>
<span class="plain-syntax">                </span><span class="identifier-syntax">TEMPORARY_TEXT</span><span class="plain-syntax">(</span><span class="identifier-syntax">prototype</span><span class="plain-syntax">)</span>
<span class="plain-syntax">                </span><span class="identifier-syntax">CompileLvalues::interpret_store</span><span class="plain-syntax">(</span><span class="identifier-syntax">prototype</span><span class="plain-syntax">, </span><span class="identifier-syntax">storage_class</span><span class="plain-syntax">, </span><span class="identifier-syntax">st</span><span class="plain-syntax">[0], </span><span class="identifier-syntax">st</span><span class="plain-syntax">[1], </span><span class="constant-syntax">0</span><span class="plain-syntax">);</span>
<span class="plain-syntax">                </span><span class="identifier-syntax">Calculus::Schemas::modify</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">schema</span><span class="plain-syntax">, </span><span class="string-syntax">"%S"</span><span class="plain-syntax">, </span><span class="identifier-syntax">prototype</span><span class="plain-syntax">);</span>
<span class="plain-syntax">                </span><span class="identifier-syntax">DISCARD_TEXT</span><span class="plain-syntax">(</span><span class="identifier-syntax">prototype</span><span class="plain-syntax">)</span>
<span class="plain-syntax">                </span><span class="named-paragraph-container code-font"><a href="8-terr.html#SP7_6" class="named-paragraph-link"><span class="named-paragraph">Add kind-checking code for run-time checking</span><span class="named-paragraph-number">7.6</span></a></span><span class="plain-syntax">;</span>
<span class="plain-syntax">            }</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        }</span>
<span class="plain-syntax">    }</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP7_1" class="paragraph-anchor"></a><b>&#167;7.1. </b>So here is the exceptional case (a) mentioned above. Suppose we have:
</p>

<blockquote>
    <p>if the lantern is bright, ...</p>
</blockquote>

<p class="commentary">where "bright" is one value of "luminance", which is both a kind of value
and also a property. We then want to test if the luminance property of the
lantern equals the constant value "bright"; and similarly for "now the
lantern is bright".
</p>

<p class="commentary"><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Handle the case of setting a property of A separately</span><span class="named-paragraph-number">7.1</span></span><span class="comment-syntax"> =</span>
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="plain-syntax">    </span><span class="identifier-syntax">property</span><span class="plain-syntax"> *</span><span class="identifier-syntax">prn</span><span class="plain-syntax"> = </span><span class="identifier-syntax">Properties::property_with_same_name_as</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[1]);</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">switch</span><span class="plain-syntax"> (</span><span class="identifier-syntax">task</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">TEST_ATOM_TASK:</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">Calculus::Schemas::modify</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">schema</span><span class="plain-syntax">, </span><span class="string-syntax">"*1.%n == *2"</span><span class="plain-syntax">, </span><span class="identifier-syntax">RTProperties::iname</span><span class="plain-syntax">(</span><span class="identifier-syntax">prn</span><span class="plain-syntax">));</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">NOW_ATOM_FALSE_TASK:</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">break</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">NOW_ATOM_TRUE_TASK:</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">Calculus::Schemas::modify</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">schema</span><span class="plain-syntax">, </span><span class="string-syntax">"*1.%n = *2"</span><span class="plain-syntax">, </span><span class="identifier-syntax">RTProperties::iname</span><span class="plain-syntax">(</span><span class="identifier-syntax">prn</span><span class="plain-syntax">));</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    }</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
</pre>
<ul class="endnotetexts"><li>This code is used in <a href="8-terr.html#SP7">&#167;7</a>.</li></ul>
<p class="commentary firstcommentary"><a id="SP7_2" class="paragraph-anchor"></a><b>&#167;7.2. </b><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Handle the case of setting a response separately</span><span class="named-paragraph-number">7.2</span></span><span class="comment-syntax"> =</span>
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="plain-syntax">    </span><span class="reserved-syntax">switch</span><span class="plain-syntax"> (</span><span class="identifier-syntax">task</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">TEST_ATOM_TASK:</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">StandardProblems::sentence_problem</span><span class="plain-syntax">(</span><span class="identifier-syntax">Task::syntax_tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">_p_</span><span class="plain-syntax">(</span><span class="identifier-syntax">PM_ResponseComparisonUnsafe</span><span class="plain-syntax">),</span>
<span class="plain-syntax">                </span><span class="string-syntax">"for complicated internal reasons this comparison isn't safe to perform"</span><span class="plain-syntax">,</span>
<span class="plain-syntax">                </span><span class="string-syntax">"and might give you a falsely negative result. To avoid what might "</span>
<span class="plain-syntax">                </span><span class="string-syntax">"be misleading, you aren't allowed to compare a response to text."</span><span class="plain-syntax">);</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">break</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">NOW_ATOM_FALSE_TASK:</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">break</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="identifier-syntax">NOW_ATOM_TRUE_TASK:</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">Calculus::Schemas::modify</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">schema</span><span class="plain-syntax">, </span><span class="string-syntax">"CopyPV(ResponseTexts--&gt;((*1)-1), *^2)"</span><span class="plain-syntax">);</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    }</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
</pre>
<ul class="endnotetexts"><li>This code is used in <a href="8-terr.html#SP7">&#167;7</a>.</li></ul>
<p class="commentary firstcommentary"><a id="SP7_3" class="paragraph-anchor"></a><b>&#167;7.3. </b>A little bit of support within Inform to help the template layer.
</p>

<p class="commentary"><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Exceptional case of setting the "player" global variable</span><span class="named-paragraph-number">7.3</span></span><span class="comment-syntax"> =</span>
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="plain-syntax">    </span><span class="identifier-syntax">nonlocal_variable</span><span class="plain-syntax"> *</span><span class="identifier-syntax">nlv</span><span class="plain-syntax"> =</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">Lvalues::get_nonlocal_variable_if_any</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">pt0</span><span class="plain-syntax">.</span><span class="identifier-syntax">constant</span><span class="plain-syntax">);</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">nlv</span><span class="plain-syntax">) &amp;&amp; (</span><span class="identifier-syntax">NonlocalVariables::must_be_constant</span><span class="plain-syntax">(</span><span class="identifier-syntax">nlv</span><span class="plain-syntax">))) {</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">schema</span><span class="plain-syntax"> = </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    }</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">NonlocalVariables::warn_about_change</span><span class="plain-syntax">(</span><span class="identifier-syntax">nlv</span><span class="plain-syntax">);</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">text_stream</span><span class="plain-syntax"> *</span><span class="identifier-syntax">exotica</span><span class="plain-syntax"> = </span><span class="identifier-syntax">RTVariables::get_write_schema</span><span class="plain-syntax">(</span><span class="identifier-syntax">nlv</span><span class="plain-syntax">);</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">exotica</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">Calculus::Schemas::modify</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">schema</span><span class="plain-syntax">, </span><span class="string-syntax">"%S"</span><span class="plain-syntax">, </span><span class="identifier-syntax">exotica</span><span class="plain-syntax">);</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">    }</span>
</pre>
<ul class="endnotetexts"><li>This code is used in <a href="8-terr.html#SP7">&#167;7</a>.</li></ul>
<p class="commentary firstcommentary"><a id="SP7_4" class="paragraph-anchor"></a><b>&#167;7.4. </b><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Make a further check that kinds permit this assignment</span><span class="named-paragraph-number">7.4</span></span><span class="comment-syntax"> =</span>
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Kinds::compatible</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">st</span><span class="plain-syntax">[0]) == </span><span class="identifier-syntax">NEVER_MATCH</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">kind</span><span class="plain-syntax"> *</span><span class="identifier-syntax">dst</span><span class="plain-syntax">[2];</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">dst</span><span class="plain-syntax">[0] = </span><span class="identifier-syntax">Kinds::dereference_properties</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[0]);</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">dst</span><span class="plain-syntax">[1] = </span><span class="identifier-syntax">Kinds::dereference_properties</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[1]);</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Kinds::compatible</span><span class="plain-syntax">(</span><span class="identifier-syntax">dst</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">dst</span><span class="plain-syntax">[0]) == </span><span class="identifier-syntax">NEVER_MATCH</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">Problems::quote_source</span><span class="plain-syntax">(1, </span><span class="identifier-syntax">current_sentence</span><span class="plain-syntax">);</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">Problems::quote_kind</span><span class="plain-syntax">(2, </span><span class="identifier-syntax">st</span><span class="plain-syntax">[1]);</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">Problems::quote_kind</span><span class="plain-syntax">(3, </span><span class="identifier-syntax">st</span><span class="plain-syntax">[0]);</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">StandardProblems::handmade_problem</span><span class="plain-syntax">(</span><span class="identifier-syntax">Task::syntax_tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">_p_</span><span class="plain-syntax">(</span><span class="identifier-syntax">BelievedImpossible</span><span class="plain-syntax">));</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">Problems::issue_problem_segment</span><span class="plain-syntax">(</span>
<span class="plain-syntax">                </span><span class="string-syntax">"In the line %1, you seem to be asking me to put %2 into %3, "</span>
<span class="plain-syntax">                </span><span class="string-syntax">"which can't safely be done."</span><span class="plain-syntax">);</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">Problems::issue_problem_end</span><span class="plain-syntax">();</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">schema</span><span class="plain-syntax"> = </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">;</span>
<span class="plain-syntax">            </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">        }</span>
<span class="plain-syntax">    }</span>
</pre>
<ul class="endnotetexts"><li>This code is used in <a href="8-terr.html#SP7">&#167;7</a>.</li></ul>
<p class="commentary firstcommentary"><a id="SP7_5" class="paragraph-anchor"></a><b>&#167;7.5. </b>Rather than just returning <span class="extract"><span class="extract-syntax">FALSE</span></span> for a generic problem message, we issue
one that's more helpfully specific and return <span class="extract"><span class="extract-syntax">TRUE</span></span>.
</p>

<p class="commentary"><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Issue problem message for being unable to set equal</span><span class="named-paragraph-number">7.5</span></span><span class="comment-syntax"> =</span>
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Rvalues::to_instance</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">pt0</span><span class="plain-syntax">.</span><span class="identifier-syntax">constant</span><span class="plain-syntax">)) {</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Kinds::Behaviour::is_object</span><span class="plain-syntax">(</span><span class="identifier-syntax">Specifications::to_kind</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">pt0</span><span class="plain-syntax">.</span><span class="identifier-syntax">constant</span><span class="plain-syntax">)))</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">StandardProblems::sentence_problem</span><span class="plain-syntax">(</span><span class="identifier-syntax">Task::syntax_tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">_p_</span><span class="plain-syntax">(</span><span class="identifier-syntax">PM_CantEquateValues</span><span class="plain-syntax">),</span>
<span class="plain-syntax">                </span><span class="string-syntax">"equality is not something I can change"</span><span class="plain-syntax">,</span>
<span class="plain-syntax">                </span><span class="string-syntax">"so either those are already the same or are different, and I "</span>
<span class="plain-syntax">                </span><span class="string-syntax">"can't alter matters."</span><span class="plain-syntax">);</span>
<span class="plain-syntax">        </span><span class="reserved-syntax">else</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">StandardProblems::sentence_problem</span><span class="plain-syntax">(</span><span class="identifier-syntax">Task::syntax_tree</span><span class="plain-syntax">(), </span><span class="identifier-syntax">_p_</span><span class="plain-syntax">(</span><span class="identifier-syntax">PM_CantChangeNamedConstant</span><span class="plain-syntax">),</span>
<span class="plain-syntax">                </span><span class="string-syntax">"I can't change that"</span><span class="plain-syntax">,</span>
<span class="plain-syntax">                </span><span class="string-syntax">"because it is a name for a constant value. Some named values, "</span>
<span class="plain-syntax">                </span><span class="string-syntax">"like 'the score', can be changed, because they were defined "</span>
<span class="plain-syntax">                </span><span class="string-syntax">"as values that vary. But others are fixed. If we write 'The "</span>
<span class="plain-syntax">                </span><span class="string-syntax">"oak tree can be sturdy, lightning-struck or leaning.', for "</span>
<span class="plain-syntax">                </span><span class="string-syntax">"instance, then 'sturdy' is a name for a value which is fixed, "</span>
<span class="plain-syntax">                </span><span class="string-syntax">"just as the number '7' is fixed."</span><span class="plain-syntax">);</span>
<span class="plain-syntax">    }</span>
</pre>
<ul class="endnotetexts"><li>This code is used in <a href="8-terr.html#SP7">&#167;7</a>.</li></ul>
<p class="commentary firstcommentary"><a id="SP7_6" class="paragraph-anchor"></a><b>&#167;7.6. </b><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Add kind-checking code for run-time checking</span><span class="named-paragraph-number">7.6</span></span><span class="comment-syntax"> =</span>
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="plain-syntax">    </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">Kinds::compatible</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[1], </span><span class="identifier-syntax">st</span><span class="plain-syntax">[0]) == </span><span class="identifier-syntax">SOMETIMES_MATCH</span><span class="plain-syntax">) &amp;&amp;</span>
<span class="plain-syntax">        (</span><span class="identifier-syntax">Kinds::Behaviour::is_subkind_of_object</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[0]))) {</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">TEMPORARY_TEXT</span><span class="plain-syntax">(</span><span class="identifier-syntax">TEMP</span><span class="plain-syntax">)</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">WRITE_TO</span><span class="plain-syntax">(</span><span class="identifier-syntax">TEMP</span><span class="plain-syntax">,</span>
<span class="plain-syntax">            </span><span class="string-syntax">"; if (~~(*1 ofclass %n)) IssueSetVariableRTP(*1, \"*?\", \""</span><span class="plain-syntax">,</span>
<span class="plain-syntax">            </span><span class="identifier-syntax">RTKindDeclarations::iname</span><span class="plain-syntax">(</span><span class="identifier-syntax">st</span><span class="plain-syntax">[0]));</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">Kinds::Textual::write</span><span class="plain-syntax">(</span><span class="identifier-syntax">TEMP</span><span class="plain-syntax">, </span><span class="identifier-syntax">st</span><span class="plain-syntax">[0]);</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">WRITE_TO</span><span class="plain-syntax">(</span><span class="identifier-syntax">TEMP</span><span class="plain-syntax">, </span><span class="string-syntax">"\");"</span><span class="plain-syntax">);</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">Calculus::Schemas::append</span><span class="plain-syntax">(</span><span class="identifier-syntax">asch</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">schema</span><span class="plain-syntax">, </span><span class="string-syntax">"%S"</span><span class="plain-syntax">, </span><span class="identifier-syntax">TEMP</span><span class="plain-syntax">);</span>
<span class="plain-syntax">        </span><span class="identifier-syntax">DISCARD_TEXT</span><span class="plain-syntax">(</span><span class="identifier-syntax">TEMP</span><span class="plain-syntax">)</span>
<span class="plain-syntax">    }</span>
</pre>
<ul class="endnotetexts"><li>This code is used in <a href="8-terr.html#SP7">&#167;7</a>.</li></ul>
<p class="commentary firstcommentary"><a id="SP8" class="paragraph-anchor"></a><b>&#167;8. </b>This never holds and there is nothing to compile:
</p>

<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">EqualityDetails::schema_empty</span><button class="popup" onclick="togglePopup('usagePopup8')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup8">Usage of <span class="code-font"><span class="function-syntax">EqualityDetails::schema_empty</span></span>:<br/><a href="8-terr.html#SP1">&#167;1</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">bp_family</span><span class="plain-syntax"> *</span><span class="identifier-syntax">self</span><span class="plain-syntax">, </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">task</span><span class="plain-syntax">, </span><span class="identifier-syntax">binary_predicate</span><span class="plain-syntax"> *</span><span class="identifier-syntax">bp</span><span class="plain-syntax">,</span>
<span class="plain-syntax">    </span><span class="identifier-syntax">annotated_i6_schema</span><span class="plain-syntax"> *</span><span class="identifier-syntax">asch</span><span class="plain-syntax">) {</span>
<span class="plain-syntax">    </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
<span class="plain-syntax">}</span>
</pre>
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